The director of public prosecutions has urged 20 environmental protesters to launch appeals against their criminal convictions after allegations that police suppressed potentially crucial evidence from an undercover police officer.

The intervention by Keir Starmer was welcomed by the protesters, who say their convictions were a miscarriage of justice.

After a three-week criminal trial and police operation costing £700,000, the 20 were convicted of plotting to break into one of Britain's biggest power stations with the aim of closing it for a week.

But their convictions were thrown into doubt after revelations that they had been infiltrated by Mark Kennedy, a police spy who was alleged to have played a central role in the organising the plot.

Starmer has written to the protesters and invited them to go to the court of appeal "as soon as possible" after studying a report by leading barrister Clare Montgomery QC, who had been commissioned by the DPP to review the safety of the convictions.

Starmer said: "The prosecution cannot lodge an appeal to the court of appeal save in very limited circumstances, which are not met here, and in my letter I have invited the defence to lodge an appeal and to include the issue of non-disclosure of material relating to the activities of an undercover police officer in any grounds of appeal."

He offered to help the protesters "in any steps necessary" to speed up the appeal, which could take months to come to court. The Crown Prosecution Service is unlikely to oppose the appeal.

Revelations about Kennedy in the Guardian earlier this year have led to four inquiries amid admissions from police chiefs and ministers that the infiltration of protest groups has gone "badly wrong".

Kennedy says he secretly taped protesters as they discussed plans to break into Ratcliffe-on-Soar power station in Nottinghamshire in April 2009.

The protesters want copies of these tapes and other reports of Kennedy's operations to help their appeal – a move the CPS would now find hard to resist.

The protesters say Kennedy's evidence would have bolstered their defence in front of the jury. They had admitted the break-in plot, but insisted they were acting to prevent the greater crimes of death and serious injury caused by climate change.

Prosecutors told the court that the protesters conspired to break into the power station as a stunt to attract publicity for their campaign against climate change.

Mike Schwarz of the London law firm Bindmans, which represents the protesters, welcomed Starmer's initiative but said: "The impression remains that the establishment have sought to undermine those campaigning against the urgent and extreme perils of climate change and, once discovered, grudgingly are conceding only as much as they have to when they have to. It has taken two years to get this far and this has only come about as a result of the persistence of climate campaigners."

When judge Jonathan Teare handed down the sentences in January, he told them: "You are all decent men and women with a genuine concern for others, and in particular for the survival of planet Earth in something resembling its present form. I have no doubt that each of you acted with the highest possible motives."

They were among 114 protesters arrested hours before the planned break-in, in what was the largest number of pre-emptive arrests of political activists in the UK.